For the life of me I can't figure how carrying a framles lump saves any energy at all. Certainly Osprey doesn't think so and they tell you just that.
I just got a 2,300 cu.in. CamelBak "Commander", a rather large daypack. Then I went out and bought two 1" wide strips of aluminum to form into a frame & bolt it in the pack W/ Stainless bolts, washers and nuts.
Finally, I bought an REI Ridgeline padded waistbelt to replace the flimsy belt that came with the pack. I'll do a little seam ripping and bar tacking of the pack's factory lumbar pad and slip the belt in behind it, just like it is on the REI pack for which the belt was made.
When finished I can do backcountry skiing in COMFORT carrying all my avy gear, food, water, 1st aid kit, extra clothes and winter survival gear.
Eric